Phosphorus-associated arsenic bioaccumulation, internal partitioning, and interfacial redistribution in a Vallisneria natans-sediment-water system.

Arsenic-phosphorus co-occurrence is an important environmental issue in eutrophic and mining-affected shallow waters, but the effects of phosphorus on arsenic distribution across sediment-water-macrophyte interfaces remain insufficiently understood. This study used Vallisneria natans as a model submerged macrophyte and combined hydroponic exposure with compartmentalized sediment-plant-water experiments to examine plant physiological responses, tissue-specific arsenic concentrations, internal arsenic partitioning, and interfacial arsenic redistribution. Exposure to 50 μg L-1 As inhibited plant growth and induced oxidative damage, whereas P addition partly alleviated these effects in an endpoint-dependent manner. After 15 d, 0.4 mg L-1 P increased the SPAD value by 29.43%, decreased MDA content by 23.41%, and increased SOD activity by 24.71% relative to the As-only treatment. Phosphorus addition decreased leaf As from 28.41 mg kg-1 to 13.31-21.53 mg kg-1. The 0.4 mg L-1 P treatment increased As allocation to the cell-wall fraction, whereas 0.8 mg L-1 P increased As in the organelle and ethanol-extractable fractions. In the sediment experiment, nominal P loading increased porewater As from 5.35 to 34.61 μg L-1, whereas overlying-water As decreased from 0.48 to 0.36 μg L-1 under the high P-loading treatment. Concurrent changes in rhizosphere pH/Eh and sediment As fractions accompanied this compartment-specific pattern. These findings show that P-associated increases in porewater As were not accompanied by increases in root/rhizome or overlying-water As, while P also altered plant physiological responses and internal As partitioning in the hydroponic experiment, highlighting compartment-specific As redistribution across the sediment-water-macrophyte system.